Under normal operating conditions, the radial and axial position(s) of the turbine blade(s) of an aeroengine jet turbine vary over a range of up to several millimeters relative to their position when the engine is cold and unloaded. Extreme radial and axial shifts and/or overspeeding of the turbine (i.e. higher than expected rates of revolution ωt) can be symptomatic of a reduction in the performance of the engine and, under certain circumstances, a degradation in its mechanical integrity. Accordingly, it is desirable to monitor the speed and position of the turbine blades relative to the engine casing in such a way that abnormal conditions may be detected within several milliseconds of their appearance, and the appropriate control actions can be implemented to bring the engine back into a more desirable operating regime and/or minimize the risk of mechanical damage.
One means to implement such a monitoring system is to install a sensor in the engine casing capable of a) detecting the presence/absence of the turbine blades b) measuring the blade pass rate, from which the rotational speed of the turbine may be inferred.
In the context of such a measurement, two quantities are important: the radial distance between the blade tips and the turbine casing (commonly referred to as the “blade-tip clearance”), and the axial position of the turbine blades relative to a fixed point on the casing. The latter may be quantified in terms of an axial offset da between the turbine blades and a fixed point on the turbine casing defined such that when the engine is cold and unloaded da=0 (see FIG. 1(a)).
The radial blade tip to casing distance dr takes a maximum value when the engine is cold and unloaded and reduces under load as a result of the combined effects of heating and centripetal acceleration of the blades. Axial shift of the blades is due to the displacement of the turbine under load. Relative to its “neutral” position in the cold, static engine, the majority of the axial shift of the turbine is toward the rear of the casing (negative shift), but a small displacement toward the front of the engine (positive shift) is also possible (see FIG. 1(b)).
A case mounted instrument capable of meeting the demands of blade sensing should preferably:                Have sufficient range to reliably detect the turbine blades when both axial shift and radial displacement are taken into consideration.        Achieve good immunity to noise and robustness to the effects of a hostile, varying temperature environment.        Achieve in-situ validation. That is, be equipped with the means to verify unambiguously that the sensor system is functioning normally.        
Conventional position sensor technologies traditionally employed for the detection of moving metallic targets (for example variable capacitance sensors, optical sensors, and eddy current devices) are unable to satisfy all of the above.
One existing attempt at the turbine blade-tip detection problem involves mounting an inductive sensor element (a coil) in the casing of the engine connected to a specially designed electronic controller (see for example US2010213929, WO2010082035 and associated applications) to drive the sensor element at a resonance frequency. In this scheme, the position of the blades is established by measuring changes to the radiofrequency (100 kHz to several hundred MHz) impedance of the inductive element which occur as a result of its interaction with the blade tips.
Such systems have been developed and have been optimized for accurate, quantitative measurements of the blade-tip clearance dr over a relatively restricted range of axial offset values da; the underlying motivation being the improvement in fuel efficiency which is known to be achievable if dr can be controlled.
However, if the requirement is not to quantify the distance dr but to simply to detect the presence and speed of the blade tips: i.e. to confirm that dr, da and ωt lie within certain “normal” operating ranges, the optimization exercise is significantly different: range rather than measurement accuracy is paramount. Accordingly, instrumentation schemes capable of delivering good performance in distance measurement applications typically fail to meet the demands of detection and speed sensing especially for the mid- and long-range detection and speed sensing of electrically conductive (though not necessarily metallic) rotor blades.
US2005/0088171 describes an eddy current sensor that drives a coil at its resonance frequency and then uses frequency demodulation to detect changes in that resonance frequency brought about by passing turbine blades. Whilst such a proximity sensor may detect fast moving blades, its sensitivity and range is low.
Therefore, there is required a rotor blade sensor that overcomes these problems.